diff --git a/src/gamedata/resourcefiles/file_wad.cpp b/src/gamedata/resourcefiles/file_wad.cpp
index f34138e62..d3aab7d5e 100644
--- a/src/gamedata/resourcefiles/file_wad.cpp
+++ b/src/gamedata/resourcefiles/file_wad.cpp
@@ -33,6 +33,7 @@
 **
 */
 
+#include <ctype.h>
 #include "resourcefile.h"
 #include "v_text.h"
 #include "w_wad.h"
diff --git a/src/intermission/intermission_parse.cpp b/src/intermission/intermission_parse.cpp
index bc1ccb7ba..5c4e0cedc 100644
--- a/src/intermission/intermission_parse.cpp
+++ b/src/intermission/intermission_parse.cpp
@@ -34,6 +34,7 @@
 */
 
 
+#include <ctype.h>
 #include "intermission/intermission.h"
 #include "g_level.h"
 #include "w_wad.h"
diff --git a/src/r_data/voxels.cpp b/src/r_data/voxels.cpp
index a9eb9eb68..8017f2c4d 100644
--- a/src/r_data/voxels.cpp
+++ b/src/r_data/voxels.cpp
@@ -35,6 +35,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <algorithm>
+#include <ctype.h>
 
 #include "m_swap.h"
 #include "m_argv.h"